PROJECT SUMMARY Severe demyelinating disorders of the central nervous system (CNS) such as multiple sclerosis (MS) impose a major burden on young lives. In MS, oligodendrocyte precursor cells (OPCs) fail to remyelinate areas of myelin damage. To date, the factors resulting in poor remyelination and repair are not well understood. Consequently, reparative therapies that benefit MS patients have yet to be developed. Microglia are the key immune effector cells of the CNS and have been repeatedly implicated in the pathophysiology of MS. On one hand, CNS microglia may be considered ?classically activated' and may produce neurotoxic and pro-inflammatory cytokines (5;6). In contrast, ?alternatively activated? microglia may clear debris after myelin damage, and offer trophic support to OPCs to facilitate regeneration (10). Here, we propose that that the lipid processing enzyme lipoprotein lipase (LPL) is a novel feature of alternatively activated microglial phenotypes that promote remyelination. LPL is the rate-limiting enzyme involved in the hydrolysis of triglyceride-rich lipoproteins (12). We have previously shown that in the peripheral nervous system (PNS), LPL is expressed in Schwann cells, macrophages, and dorsal root ganglia neurons, and its abundance and activity is increased following nerve crush injury (28;29), suggesting that LPL may scavenge and reutilize myelin-derived lipids. While, we postulate that LPL has a similar function in CNS microglia this has never been empirically determined. Our preliminary studies have shown that LPL expression and abundance is associated with alternatively activated microglia, lipid up- take, and active remyelination. Taking our initial observations into account, we hypothesize that LPL is a novel feature of a microglial phenotype that actively supports remyelination and repair through the clearance of lipid intermediates and myelin debris. To test our hypothesis, in specific AIM I we will explore the role of microglial-LPL in vivo by specifically knocking out LPL in CNS microglia. We will immunize these mice with a MOG peptide to induce experimental autoimmune encephalomyelitis (EAE), and compare remyelination processes in microglial-specific LPL deficient mice versus wild-type controls. We expect that depleting LPL from CNS microglia will inhibit or delay central remyelination processes. In AIM II we will interrogate the mechanism of action, using a novel cell line to delineate whether LPL enzymatically degrades and/or phagocytoses myelin- derived lipids. We will also demyelinate ex vivo brain slice cultures with lysolecithin in order to explore the role of LPL during remyelination in real time, and to determine the feasibility of pharmacological LPL activators to promote remyelination. The proposed study will serve to develop a conceptually novel program of research that will not only uncover the fundamental mechanisms underlying the role of LPL in microglial function, but will also guide studies assessing the feasibility of LPL as a novel reparative therapy to improve clinical symptoms in MS.